The present invention relates to a two color electrostatic copying machine which is selectively operative for one color positive or negative copying.
A novel and unique two color electrostatic copying machine is disclosed in my copending U.S. Pat. application Ser. No. 912,273, filed June 5, 1978, entitled "COLOR ELECTROSTATOGRAPHIC PROCESS AND MATERIAL FOR PRACTICING SAME". The present invention constitutes improvements to my basic copying machine which enable it to be operated in not only a two color copying mode, but also in a one color positive or negative copying mode in either of the two colors.
Color electrostatic copying machines which produce full color copies are known in the art. These are generally of two types. The first type comprises a single photoconductive drum or belt which is exposed to a light image of an original document three times through filters of three primary colors respectively. After each imaging operation, a toner substance of a corresponding color is applied to the drum to form a color toner image which is transferred to a copy sheet. In this manner, three color toner images are sequentially formed on the drum and transferred to the copy sheet in register to produce a color copy. Often, a fourth black toner image is formed and transferred to the copy sheet in register with the three color toner images.
In such a copy machine it is essential that the toner images be transferred to the copy sheet in perfect register. The control mechanism for such a copying machine is therefore intricate and expensive. The three or four imaging operations for each copy require a disproportionate amount of time, making the process very slow.
The second type of color copying machine is much faster in operation but also much more expensive to manufacture. Such a copying machine comprises three or four photoconductive drums or belts. The original document is passed over all of the drums in one scanning movement, sequentially imaging the drums through three respective primary color filters. A toner development unit is associated with each drum. The copy sheet is fed through the machine in one pass, with the toner images being transferred thereto in register through sequential engagement with the drums.
In addition to the increased cost of the three or four drums compared to only one drum or belt in the first type of color copying machine, an intricate mechanism is also required in the second type of machine to ensure perfect register of the three of four toner images on the copy sheet.
A full color copying machine is unnecessary in many business operations where only commercial documents are copied, since such documents generally only comprise the colors black and red, in addition to a white background. This is because accounting records and the like generally contain credit entries in black and debit entries in red. Since in many such documents the debit and credit entries may be distinguished from each other only by the color of ink, many offices have purchased or leased full color copying machines for copying such records. The full color copying capability is wasted since it is only necessary to distinguish red from black on the copies.
In addition it is often desired to make copies in only one color. Although full color copying machines can make black and white copies from black and white originals through color addition, the quality is generally not as good as that produced by a conventional black and white copying machine. A full color copying machine is not at all capable of producing black and white copies from colored originals which are required in various applications. It is further disadvantageous to produce black and white copies using a full color copying machine due to the high consumption of expensive colored toner.